1. Field of the Invention
The present invention relates to a projection lens, particularly to a projection lens suitable for a rear projection type image display apparatus.
2. Description of the Related Art
“A rear projection type image display apparatus”, in which a white light beam emitted from a white light source is separated into color components of three primary colors, a transmission type or a reflection type of image display device is lit with each separated color component light beam, each color component light beam transmitted through or reflected from each color component of the image display device is combined with a color combining system, and a color image is displayed by performing projection on a transmission type screen, has been known as a liquid crystal projection TV and the like.
FIG. 24 is a sectional view schematically showing an example of “the rear projection type image display apparatus”. An optical unit b is provided in a cabinet a. A transmission type screen c is provided at a front surface of the cabinet a. The light beam from the optical unit b is projected onto the transmission type screen c through a first folding mirror d, a projection lens e, and a second folding mirror f. Therefore, depth reduction of a TV main body and upsizing of the screen are realized.
FIG. 25 is a schematic block diagram showing an example of an optical system in a well-known three-panel type liquid crystal projection TV. In FIG. 25, the numeral 1 represents a white light source, the numeral 2 represents a UV-IR cut filter, the numeral 3 represents a dichroic filter through which blue light is transmitted, the numeral 4 represents a dichroic filter through which red light is transmitted, the numerals 5, 6, and 7 represent reflection mirrors, the numeral 8 represents a liquid crystal light valve for displaying the blue light, the numeral 9 represents a liquid crystal light valve for displaying green light, the numeral 10 represents a liquid crystal light valve for displaying the red light, the numeral 11 represents a color combining prism, the numeral 12 represents a projection lens, and the numeral 13 represents a screen.
The light beam emitted from the white light source 1 is separated with “a color separation system” including the dichroic filters 3 and 4 and the reflection mirrors 5, 6, and 7, the corresponding liquid crystal light valves 8, 9, and 10 which are the transmission type image display devices are lit with the separated color component light beams respectively, the color component light beams are combined with “the color combining system” including the color combining prism 11 to be incident to the projection lens 12, and a display image of each liquid crystal light valve is magnified and combined to be focused and projected on the screen 13.
Recently, the depth reduction of the projection TV main body and the upsizing of the display screen, i.e., the projection of the image onto a large screen with short projection distance, are demanded for the liquid crystal projection TV, so that a short focal distance and a wide angle of view are required for the projection lens. In order to satisfy such demands, it is necessary to increase a ratio of a back focus to the focal distance of the projection lens. On the other hand, in order to magnify and project the image on the transmission type image display device onto the screen with high contrast, it is necessary to utilize a luminous flux outgoing from the transmission type image display device at an angle substantially perpendicular to the transmission type image display device. Therefore, it is necessary that the projection lens has telecentric characteristics so that a principal ray of an off-axis of the projection lens is perpendicular to the transmission type image display device.
Unlike a CRT (Cathode Ray Tube) method, in the liquid crystal light valve, distortion aberration can not be electrically corrected due to dot matrix display. Accordingly, it is necessary that the small distortion aberration is realized by the projection lens itself. However, the realization of the small distortion aberration in the projection lens becomes hinders in realizing the wide angle of view and the long back focus of the projection lens.
In consideration of the above-described problems, there has been proposed a conventional technology described in Japanese Patent Application Laid-Open (JP-A) No. 2003-156683. In a first embodiment of JP-A No. 2003-156683, the aberrations are corrected by using two aspherical lenses made of acryl resin in a nine-lens configuration.
However, in the case of the above-described acryl resin lens, there is a problem that positional shift of the back focus (change in focusing surface) due to temperature change is large. For example, assuming that the amount of change in refractive index of the acryl resin by the temperature change is set to −1.088×10−4/° C., the change in focusing surface is generated as large as +31.6 μm when the temperature is increased by 20° C.
Further, as pixels of the transmission type image display device becomes high density (finer), improvement in optical performance of the projection lens becomes important, particularly, a decrease in “chromatic aberration of magnification” becomes important. However, in the conventional projection lens, the chromatic aberration of magnification has +19 μm at the blue light (450 nm) and the chromatic aberration of magnification has +22 μm at the red light (620 nm). Since these values are larger than a pixel size of a 15 μm by 15 μm square of the current image display device, there is a problem that color shift is generated toward a periphery in the projected image.